This invention relates in general to a generalized active disturbance rejection approach for electrical power steering (EPS) systems. In particular, this invention relates to an active disturbance rejection circuit and control algorithm having a minimal effect on steering feel. This invention further reduces the dependency on accurate disturbance frequency information in operation to maintain consistent steering feel and compensate the dynamics differences and potential interactions with other functions in EPS or other vehicle systems.
Methods exist to provide certain levels of active disturbance rejection through either disturbance feed-forward cancellation or attenuation of system gain within an interested frequency range via certain resonant filters. In general, a disturbance feed-forward system detects the disturbance and provides this disturbance information to an actuator to counteract the deleterious effects of the detected disturbance. This typically involves two steps, namely, disturbance detection and command generation. However, disturbance detection typically relies on a certain amount of transient time in order to accurately detect the disturbance. This approach might not be applicable in some scenarios, such as brake pulsations, where the disturbance frequency is proportional to the wheel speed and changes during the braking process due to the change in wheel speed. In addition, the disturbance detection process is typically realized via different types of filters.
In EPS applications, the resonant filter will also filter out the same frequency component that exists in the original motor torque. This filtering will affect the steering feel and require a tedious, iterative calibration process to balance steering feel and active disturbance rejection capability. Due to this condition, the disturbance detected not only includes the original disturbance but also includes the signals from original motor torque. Because of the inclusion of overlapping frequencies from different sources, this disturbance rejection method affects steering feel because the system will also cancel the frequency component of the original motor torque. While the disturbance rejection capability can be increased via high gain for either proportional method and/or integral method, this, in turn, will make the system more unstable and sensitive to parameter uncertainties. Also, the implementation of these prior art methods requires significant computational resources and memory due to their use and implementation of trigonometry. There is also a tendency to introduce complexity in analyzing a system's performance in terms of stability and effectiveness because the system is inherently nonlinear. This complexity is further compounded due to the level of accuracy needed in identifying the disturbance frequency in order to detect the disturbance.
Attenuating system gain within a disturbance frequency range is another typical approach in existing literatures. The strategy here is to lower the system gain such that the disturbance energy is less perceivable at the steering wheel or other EPS component. This real-time strategy is intended to respond instantaneously to disturbances and, therefore, be applicable to scenarios such as brake pulsation compensation. However, this approach cannot achieve 100% disturbance rejection, even from theoretical point of view. Also, it will affect the steering feel and stability significantly and often requires re-tuning of the original system.
Other methods attempt to utilize the concept of a standard feed-forward approach (in reference to control input, meaning that disturbance is added into control channel instead of being fed back to construct tracking error as feedback control) to cancel disturbances, as shown in FIG. 1A. Tassist is the total assist in rack coordinate without active disturbance rejection capability. Tfiltered is the active disturbance rejection torque in rack coordinate. Tmot is the final assist torque in rack coordinate. This idea is fairly standard and well-known within controls community. However, because an external disturbance, Tdisturbance, is not measurable at its generation source and is measured indirectly at the output, which is to be controlled/affected by control input, this standard feed-forward approach is essentially a feedback configuration. These approaches typically fail to achieve 100% disturbance reduction due to an inherent feedback mechanism in the loop: column torque-motor torque-column torque. Since the disturbance is indirectly measured via the column torque sensor, disturbance rejection algorithms are feedback mechanisms which inherently will not provide 100% disturbance rejection. A high-gain compensation approach may be utilized to reduce such effects but tends to make the system overly sensitive to parameter uncertainties and high frequency noises from column torque sensor which generates instability issues. Furthermore, this approach will have impact on steering feel due to the fact that the feedback compensator, as shown in FIG. 1A. The feedback compensator is typically represented as resonant filter or a modulation-demodulation process. These devices will filter out not only the disturbance but also the related signal in Tassist that contains a similar frequency to that of the disturbance. This affects steering feel and requires an iterative tuning process to achieve the improved steering feel while rejecting the disturbance.
FIG. 1B is a representation of a standard EPS system where the variables TDriver, Tassist and Tdisturbance are defined as driver torque, assistance torque and disturbance force (in an appropriate coordinate system) applied to the rack. Other coordinates can also be chosen when necessary. In prior art EPS systems, disturbance forces are typically not measured and are considered difficult to measure directly. These forces, however, are indirectly observable from other available EPS inputs, such as column torque, column velocity, motor velocity, etc. In the above example, the disturbance is defined as a rack force disturbance and the measurement state is defined to be column torque.
There are, in particular, two known prior art systems, disclosed in US Patent Publication No. 2012/0061169 to General Motors and U.S. Pat. No. 8,219,283 to Ford Motor Co., which illustrate aspects of the prior art described above. The disclosures of these references are incorporated by reference.